1. Field of the Invention
The present invention relates to a multi-hop relay system, and, in particular, to an apparatus and method for frequency reuse to avoid interference between a Relay Station (RS) and a Mobile Station (MS).
2. Description of the Related Art
To realize a self-adaptive wireless network required in a next generation mobile telecommunication system, techniques applied in an ad hoc network need to be introduced to the mobile telecommunication system. A typical example of the mobile telecommunication system is a multi-hop relay based broadband wireless access communication system which is constructed with a fixed Base Station (BS) and employs a multi-hop relay scheme used in the ad hoc network.
In the broadband access communication system, communication between the BS and a Mobile Station (MS) is achieved through one direct link. Thus, a highly reliable wireless communication link can be readily constructed between the MS and the BS. However, since a position of the BS is fixed, configuration flexibility of a wireless network is low. As a result, it has been difficult to effectively provide service in a wireless communication environment where there is a rapid change in traffic distribution and call demands.
To address this shortcoming, the broadband wireless access communication system employs a relay scheme in which data is delivered in a multi-hop manner by using a plurality of surrounding MSs or fixed BSs. According to the multi-hop relay scheme, a network can be rapidly reconfigured in response to a change in the surrounding environment, and the entire wireless network can be further effectively managed. Therefore, the self-adaptive wireless network required by next generation wireless telecommunication systems can be realized by modeling after the multi-hop relay based broadband wireless access communication system.
Advantageously, the multi-hop relay scheme can cover a partial shadow area resulted from an insufficient electric field. In addition, the use of the multi-hop relay scheme can reduce an initial installation charge since a relay is established in an initial stage when a service request is not frequently made. Therefore, the multi-hop relay scheme can broaden cell service coverage and increase system capacity.
Now, referring to FIG. 1, a communication method using a Relay Station (RS) in a broadband wireless access communication system employing a conventional multi-hop relay scheme will be described.
An MS3 111 within the coverage area of a BS 101 is connected to the BS 101 through a direct link. An MS1 107 and an MS2 109 are located outside the coverage area of BS 101, and thus have a poor channel quality with respect to BS 101. Therefore, MS1 107 and MS2 109 are connected to BS 101 though a relay link by use of an RS1 103 and an RS2 105, respectively. When MSs 107 and 109 communicate with BS 101 when MSs 107 and 109 are located outside the coverage area of BS 101 or in a shadow area suffering a serious shielding phenomenon due to buildings, a better wireless channel quality of a link between MSs 107 and 109 and BS 101 can be achieved by the use of RS1 103 and RS2 105. Therefore, by using the multi-hop relay scheme, BS 101 can provide a high-speed data channel in a cell boundary region having a poor channel quality and also can extend the cell service coverage.
In the broadband wireless access communication system employing the multi-hop relay scheme, communication using RS1 103 and RS2 105 is made using a frame which has two transmission periods (hereinafter referred to as phases) in a time axis. In a first phase (i.e. PHASE-I) of the frame, a BS-RS transmission or a BS-MS transmission is achieved through a direct link. In a second phase (i.e. PHASE-II) of the frame, a frequency is reused for an RS-MS transmission for a relay communication. In the relay communication, information transmitted from the BS to the RS during the PHASE-I is retransmitted to the MS. In PHASE-II, power transmitted from a certain RS acts as a signal component (indicated by a solid line) for its corresponding MS and acts as interference (indicated by a dotted line) for a neighboring MS. For example, as shown in FIG. 2A, the frame may have two transmission periods, that is, a PHASE-I 201 and a PHASE-II 203. Alternatively, as shown in FIG. 2B, the frame may have three transmission periods, that is, a PHASE-I 205, a PHASE-II 207 and a PHASE-I 209. As such, in the multi-hop rely type wireless access communication system using RSs, it is possible to increase the capacity of a packet transmission system by reuse of wireless resources.
However, as shown in FIG. 3, if a frequency is reused in a link between an RS and an MS (e.g., a link between an RS1 301 and an MS1 305, a link between an RS2 303 and an MS2 307, etc.), the MS1 305 may receive a strong interference signal from its neighboring RS2 303 when the MS1 305 is located in a boundary coverage of the serving RS1 301. In this case, in order to increase cell capacity, a new technique may be taken into account in which the delivery of transmission power from the RS1 301 to the MS1 305 is stopped to avoid signal interference between two links RS1-MS1 and RS2-MS2. In order to accommodate the technique, a procedure for a method of frequency reuse has to be defined to avoid interference between an RS and an MS in a multi-hop relay system.